The pineal hormone melatonin and its major urinary metabolite 6-hydroxymelatonin are being quantified by gas chromatography-negative chemical ionization mass spectrometry. Plasma melatonin levels in humans have been assayed and reveal that nighttime melatonin release is inhibited by light of daylight intensity. Urinary melatonin metabolite assays have indicated the normal range for daily adult excretion as well as abnormal patterns in patients with diminished adrenergic activity. Studies are in progress to determine the pattern of excretion during normal human growth and development, particularly with regard to puberty and the menstrual cycle. Animal studies are being pursued to determine the contribution of extra-pineal sites of melatonin synthesis to urinary 6-hydroxymelatonin.